1. Technical Field of the Invention
The present invention relates to a coating method for coating an internal member having holes in a vacuum processing apparatus and the internal member having holes coated by the coating method. In detail, the present invention relates to a method for forming a ceramic coating membrane on a internal member having its surface with small holes (pores) in a vacuum processing apparatus used for producing semiconductor wafers, in order to improve the durability of the member. As the internal member having holes, for example, there is an electrostatic chuck having a suction surface for holding a semiconductor wafer. Accordingly, the present invention also relates to the electrostatic chuck coated by the above coating method.
2. Description of the Related Art
When applying CVD process, sputtering, etching, etc. on a semiconductor wafer, an electrostatic chuck has been widely used to hold the semiconductor wafer in view of effecting the designated processing appropriately.
Basically, the electrostatic chuck has a structure where an electrode of conductive material is embedded. In operation, when a high-voltage direct current (DC) voltage is applied to an electrode layer, an electrostatic absorbing force is generated on a suction surface of an insulating body of the electrostatic chuck.
However, if the suction surface of the insulating body is worn by the semiconductor wafer or if sputtering material of the sputtering process collides with the suction surface, there may arise problems that the suction surface is damaged in its insulation performance and the durability of the insulating body is deteriorated.
In order to solve the problems, there is known a technique as follows.
For example, Japanese Patent Publication (kokai) No. 7-335732 discloses a method of forming a coating film of ceramic material, such as Al2O3, on the suction surface of an electrostatic chuck by means of plasma spraying. This coating film serves to protect a member having the film formed thereon. Note, the insulating body itself may be formed by a coating film of ceramic material.
Additionally, it is also known that when applying a variety of above-mentioned treatments (processings) to a semiconductor wafer absorbed by an electrostatic chuck, the temperature of the semiconductor wafer has a great influence on the quality of the processed wafer. Therefore, Japanese Patent Publication (kokai) No. 7-335732 further proposes a technique to blow temperature-regulated He-gas against the suction surface of an electrostatic chuck in order to control the temperature of a semiconductor wafer to be absorbed by the chuck. In this case, the electrostatic chuck is provided, on the suction surface, with gas injection holes.
In order to form the above-mentioned coating film on the electrostatic chuck having the gas injection holes in the suction surface, it is required for a coating material so as not to enter the gas injection holes. Nevertheless, there is no industrial method which is effective to this the requirement up to this day.
For example, there may be expected a method of applying adhesive tapes to the gas injection holes in the suction surface. In accordance with this method, however, since each adhesive tape conceals even the suction surface (portion) around the gas injection hole, a zone having no coating film is produced in the circumference of each gas injection hole.
Alternatively, it is also supposed to insert padding plugs of fluoropolymer having low adherability to the coating film into the gas injection holes, respectively. However, since each of the padding plugs has a small diameter, the resinous pudding plugs may be molten due to heat of plasma spraying during the coating process thereby causing their inferior closing of the gas injection holes or causing the molten resinous material to drop into the gas injection holes for fixation.
It is noted that the gas injection holes are communicated, on the backside of the above-mentioned insulator, with a gas supply passage built in an electrostatic chucking unit. It is extremely difficult to remove the resinous material of each padding plug, which has dropped into the gas supply passage through the gas injection hole and which is firmly fixed in the gas supply passage. Providing that even trace amounts of resin is remained, when applying a CVD process etc. to a semiconductor wafer while using an electrostatic chuck, the residual resin will be vaporized to influence the quality of the processed wafer.
If the padding plugs are made of a metal material, they are not molten by heat of plasma spraying. However, it is noted that coating material conjugates to the metal material of the padding plugs disadvantageously. Even if it is desired to extract the padding plugs after the coating process, they could not be extracted with ease since they are welded to the coating film. High-handed extraction would cause the coating film to be peeled or cracked.
Meanwhile, there is a case that the coating film has already contained microscopic cracks before extracting the padding plugs from the gas injection holes. As for the reason of the microscopic cracks, we assume that a difference in thermal deformation between the padding plugs and the coating film, both of which have been subjected to thermal expansion due to the heat of plasma spraying and the sequent cooling, produces an excessive thermal stress therebetween to make the coating film contain the above defects, such as cracks. It is also noted that the more strongly the padding plugs are welded to the coating film, the larger the above thermal stress grows.
The above-mentioned problem arises in not only the electrostatic chuck but a internal member having holes which is disposed in each processing chamber of all kinds of vacuum processing apparatuses and which has small holes formed in the member's surface to be covered with a coating film.